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V v.-.. 



Tfl 439 
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AIN AND REINFORCED 

CONCRETE 



BY 



WILLIAM O. LICHTNER 




PLAIN AND REINFORCED 

CONCRETE 

HAND BOOK 

BY 

WILLIAM O. LICHTNER 

ASSOC. M. AM. SOC. C. E. 

FOR USE IN 

LECTURES AT EVENING SCHOOL OF 
BUILDING TRADES 

OF THE 

MASS. CHARITABLE MECHANIC 
ASSOCIATION 



This book not for sale except 
by permission of the author 



PRINTED FOR 

M. C. M. A. TRADE SCHOOL 

BOSTON, MASS. 

1914 



V 



Copyright 1914 
By William O. Lichtner 



m 14 1914 



THE SEAVER-HOWLAND PRESS 
BOSTON, MASS. 

©CI,A369331 



PREFACE 

This book is written as a text book for use of the students 
taking the course in Plain and Reinforced Concrete at the 
Massachusetts Charitable Mechanic Association Trade School. 
The wording has been made as elementary as possible, and 
only such technical terms have been employed as are generally 
used on actual construction work. 

The subject of Plain and Reinforced Concrete has been 
divided into chapters, each chapter developing the subject 
by first taking up in detail how to select and test the materials 
of which concrete is composed and then taking up the proper 
ways and means of combining them to produce a first class 
concrete. Each chapter is followed by a series of questions for 
the purpose of assisting those students who wish to review the 
amount of real knowledge which they have derived from a 
study of the chapter by answering the questions fully without 
referring to the text. 

The author wishes to state that he was assisted in this 
work by Mr. Frank A. Baker, who figured the tables, made 
the drawings, took a number of the photographs, and vis6d 
the manuscript. He wishes to thank the following firms for 
their kindness in furnishing some of the photographs: — Wilson 
and Tomlinson, Benjamin Fox, Inc., Edward A. Tucker, and 
Sanford E. Thompson. 

For a more technical treatment of the subject of Plam and 
Reinforced Concrete the author advises the well-known work 
of Taylor and Thompson, ''Concrete Plain and Reinforced/' 
Another splendid book by the same authors on cost of concrete 
work, titled '' Concrete Costs, " is recommended to those who 
are interested in estimating, contracting, etc. 

THE AUTHOR 



CONTENTS 



CHAPTER I. MATERIALS FOR PLAIN CONCRETE 

Page 

Definition of Plain Concrete 1 

Definition of Portland Cement 2 

Kinds of Cement 3 

Setting of Cement 3 

Normal and Abnormal Cements 3 

Packing Portland Cement 4 

Storing Portland Cement 4 

Definition of Fine Aggregate or Sand 5 

Kinds of Sand 5 

How TO Test for a Clean Sand ^ 7 

Method of Using Dirty Sand 7 

Definition of Coarse Aggregates 9 

Crushed Stone vs. Gravel for Coarse Aggregate 9 

Natural Mixtures of Bank Sand and Gravel 9 

Screenings from Crushed Stone 10 

Water 10 

CHAPTER 11. PROPORTIONING OF MATERIALS 

Theory of Proportioning 12 

Kinds of Mixes 13 

Amount of Cement vs. Strength of Concrete 14 

Use of Very Fine Sand 14 

Screening Bank Gravel 14 

Volume of Concrete vs. Volume of Aggregate 15 

CHAPTER III. MIXING CONCRETE. 

Specifications for Hand Mixing 17 

Tools and Apparatus 18 

Sizes of Gangs 19 

Method of Measuring 21 

Batch Sizes 22 

Consistency of Mix 24 

Density 25 

V 



CONTENTS 
CHAPTER IV. PLACING OF CONCRETE. 

Page 

Placing Concrete 27 

Placing Concrete in Forms 27 

Placing Concrete Under Water 27 

Protecting Concrete in Hot Weather 28 

Placing Concrete in Freezing Weather 28 

CHAPTER V. REINFORCED CONCRETE 

Definition of Reinforced Concrete 31 

Use of Steel in Concrete 31 

Kinds of Reinforcing Steel 34 

Corrosion of Metal Reinforcement 36 

Bond Between Steel and Concrete 36 

Quality of Reinforced Steel 36 

Fireproofing 36 

Joints 37 

Precautions 37 

CHAPTER VI. FORMS FOR CONCRETE 

Design of Forms 39 

Size of Form Board 40 

Thickness of Form Board 40 

Width of Form Boards 41 

Length OF Form Boards 41 

Green vs. Seasoned Lumber 41 

Kinds of Lumber 42 

Square Edge vs. Tongue and Groove 42 

Planed vs. Rough Lumber 42 

Dimensions of Cleats, Studs, and Posts 43 

Grade of Lumber for Form Work 43 

Oiling Surfaces of Forms Coming in Contact with the Concrete . 44 

Kinds of Oil 45 

Method of Supporting Forms 45 

Removing Forms 45 

CHAPTER VII. SURFACE FINISH. 

Rubbing Surface with Brick 47 

Picking Surface 47 

Plastering Surface 49 

Washing Surface with Pure Cement Mortar 49 

vi 



CONTENTS 
CHAPTER VIII. CONCRETE DESIGN 

Page 

Walls ". 53 

Cellar and Basement Walls 54 

Walls above Cellar and Basement 55 

Small Reinforced Concrete Structures 57 

Table for Designeng Reinforced Concrete Beams and Slabs . . 59 

Sidewalks 61 

Foundations for Sidewalks 61 

Retaining Walls 62 

Dams 63 

Fence Posts 65 



vu 



CONCRETE DATA 



Portland Cement weighs per bag, net 94 lb. 

'' barrel, net 376 '' 

Loose Portland Cement diYtrdiges^^r cubic ioot ?iho\it . 92 ^* 

Weight of Concrete per cubic foot 

Cinder Concrete averages 112 " 

Sandstone Concrete averages 143 ^' 

Limestone Concrete averages 148 '^ 

Gravel Concrete averages 150 ^* 

Trap Concrete averages . . 155 '^ 

-A'derage quantity of concrete mixed, wheeled 50 ft, and placed in 
10 hours by 3 men 180 cu. ft. 



Add for each additional man in gang . . 
Average load of concrete in iron wheelbarrow . . . 
Average load of sand or gravel in iron wheelbarrow 

Average load of a one-horse dump cart 

Average load of a two-horse bottom dump wagon . . 
Amount of cement 

Per cu. yd. of concrete . . . . 1 : 1| : 3 mix . 



60 
2 
3 

13 

30 



8.0 bags 



H ii ii ii ii 




1:2 :4 " 


. . . 6.3 


ii 


H ii ii ii ii 




1 : 2i : 5 '■ 


. . . 5.2 


ii 


ii ii ii ii ii 




1:3 :6 " 


. 4.4 


ii 


Amount of sand 










Per cu. yd. of concrete 




: 1^ : 3 mix . 


. . 11.3 cu 


ft 


ii ii a a a 




2:4" . 


. . 11.9 " 


ii 


a it ii ii a 




2|:5 " . 


. . 12.4 " 


ii 


a ii a a a 




:3 :6 " . 


. '. 12.7 " 


ii 


Amount of broken stone 










Per cu. yd. of concretj . 




1§ : 3 mix . 


. 22.7 cu. 


ft 


a ii a a a 




:2 :4 '■ . 


. . 23.8 " 


ii 


ii ii a a a 




2|:5 " . 


. . 24.6 " 


ii 


a a ii ii ii 




3 :6, " . 


. . 25.4 " 


i( 



IX 



PLAIN AND REINFORCED 

CONCRETE 

CHAPTER I 

MATERIALS FOR PLAIN CONCRETE 

Definition of Plain Concrete. Plain concrete is artificial 
stone made by mixing together pieces of broken stone or gravel 
with clean sand and cement, using enough water to make a 
mushy mixture. 

The cement, mixed with water, is the material which unites 
all the particles of sand with the broken stone or gravel. This 
mass of mixed materials begins to stiffen in about half an hour 
and in from 10 to 24 hours the mass becomes hard enough so 
that an impression cannot be made readily by pressing on it. 
In a month's time the entire mass is practically a hard stone. 

The broken stone or gravel is known as the Coarse Aggregate 
and the sand is known as the Fine Aggregate. 

Concrete is always indicated on drawings as shown in Fig. 1, 
page 2. The triangular pieces in the figure indicate the broken 
stone, the round pieces indicate the sand, and the dots indicate 
the cement. 

The theory of concrete is that the large broken stones or 
gravel making up the mass of the concrete would have a great 
number of voids or air spaces between them provided there 
were no smaller stones. The smaller stones fill up some of the 
spaces between the larger ones, but still leave numerous voids 
or spaces unfilled. These in turn are supposed to be filled with 
the sand. The cement, mixed with water, coats all of these 
particles from the largest broken stone down to very fine sand 
grains, and acts as a binding agent. The mass if allowed to 
remain in this state for several hours begins to stiffen, due to 
the cement and water undergoing a chemical change which 
produces the gluing or cementing qualities. In order to 
produce a very strong concrete every particle, no matter how 
fine, must be coated with the wet cement. A well made piece 
of concrete is like two sticks which are properly glued together. 
It is a fact that a glued stick will split anywhere rather than at 
the joint if the joint is properly made. The same thing is true 



2 PLAIN AND REINFORCED CONCRETE 

with a well made piece of concrete; if hit with a sledge it will 
break through the stones instead of knocking the stones loose 
by breaking the bond around them. 

Definition of Portland Cement. Portland cement is a manu- 
factured product made by mixing limestone or chalk (which is 
nearly pure limestone) together with clay or shale. The correct 
proportions in which to mix these two materials are deter- 
mined by a chemical analysis. These materials are then ground 
to a very fine powder and fed, together with powdered coal, 
into long rotary kilns. The mixture is then burned at a tem- 
perature of about 3,000 degrees Fahr. and forms into a dark, 
porous stone called cement clinker. This cement clinker is 
cooled, crushed, and ground again to a very fine powder, which 
changes from the dark color to the very light gray -color which 
is characteristic of Portland cement. 






lo 



* .O O' 4 



. o . ^' . O . O ' o / ^» o« »o . «. . 

Fig 1.— Sectional View of Concrete. (See page 1.) 

After the cement is ground it is placed in large storage stock 
houses, where it must remain for a while to season before it is 
put into bags or barrels for shipment. The cement after being 
ground is called green cement and is not fit for use, but after 
remaining in the storage stock house for a short time, it seasons 
and is ready for use. 

To illustrate how fine Portland cement really is: From 
75% to 85% of the cement will pass through a screen having 
200 openings to the inch each way, or 40,000 openings to the 
square inch. From 92% to 98% will pass a screen having 100 
openings to the inch each way, or 10,000 openings to the square 
inch. One is not likely to get any brand of standard Portland 
cement that will not be as fine as just indicated, for all the 
cement companies comply with the standard specifications for 



MATERIALS FOR PLAIN CONCRETE 3 

testing cement as issued by the American Society for Testing 
Materials, the American Portland Cement Manufacturers, the 
United States Government, etc. 

Kinds of Cement. There are a number of different kinds of 
cement that are used on construction work, but the most com- 
mon of these, Portland cement, is the only one that need be 
considered. The layman will not ordinarily have use for the 
other kinds of cements, such as Natural cement. Slag cement, 
and Puzzolan cement. Under the head of Concrete Finish, 
a white finish cement will be described, but this is only used 
for ornamental purposes. 

Setting of Cement. There are two terms which are used to 
indicate stages of hardening of the Cement after water has 
been added to it to form a paste. As soon as this paste ceases 
to be plastic the cement is said to have its Initial Set, This 
first stage of hardening is the most important, and after cement 
has reached this stage it must not be disturbed, for if it is the 
cement will show a loss of strength in the final hardening or 
even have no strength at all. In good Portland cement the 
Initial Set will not occur within less than one-half hour, which 
will give plenty of time to mix and place the concrete before 
the cement has commenced to harden. The second stage of 
hardening is called the Final Set, This Final Set has taken 
place when the cement has acquired such a degree of hardness 
that only a slight impression will be made on the surface of 
the concrete when, the thumb nail is pressed lightly upon it. A 
cement should receive its Final Set in less than ten hours. 

Normal and Abnormal Cements. All Portland cements 
which pass the standard specifications are called ^'normal,'' 
while any cement which does not pass these specifications in 
one or several ways is called '' abnormal." Never use abnormal 
cement. 

Abnormal Cements. Causes of abnormal cement. The causes 
of an abnormal cement may be due to (a) shipment in a leaky 
railroad car; {b) storage in damp places; {c) storage or ship- 
ment during long spells of extremely hot weather. In the case 
of {a) and (6) the cement becomes lumpy or may even be caked 
^ hard so that the lumps cannot be crumbled by pressure between 
the fingers. The lumpy cement should not be used. A bag of 
cement which is caked must never be broken up and used. In 
the case of item {c) the cement may develop a ''flash set." 
This occurs most frequently in extremely hot weather. Cement 



4 PLAIN AND REINFORCED CONCRETE 

has a flash set when it stiffens or hardens in less than a half 
hour^s time after adding water. It can readily be seen why a 
flash set cement should not be used, for it would not give 
time enough to get the concrete mixed, wheeled to the place 
where it is to be deposited, and spread and tamped, before it 
had started to stiffen. The stiffening of the soft concrete 
indicates that the cement has started to get its initial set. If 
the mass is now disturbed a part of the gluing or sticking prop- 
erty of the cement is destroyed, so that the resulting concrete 
will not be strong. 

When the concrete is machine-mixed sometimes a flash set 
is not detected, as the cement flashes in the mixer and if the 
machine is kept running the mass will be broken up after the 
cement has flashed. If an excess of water is used in the beginning 
this water will mix with the cement and the cement will be 
''regaged. '' Such concrete is very dangerous to use, as it is 
doubtful whether it will harden properly even after a long 
interval of time. There is little danger of not noticing the 
flashing of the cement in the mixer if it is watched by the man 
at the machine. Besides this, when the mass breaks up more 
water will have to be added in order to regage the cement and 
bring the concrete back to a mushy mixture, thus attracting 
the attention of the man running the machine. 

While it is not safe to use a cement which flashes, the cement 
can be kept for a while, and it may possibly become normal 
again. 

In order to determine whether a cement has a flash set, take 
a small quantity on a glass plate and having formed it into a 
crater shape at the top, pour in water. Mix this thoroughly for 
about 2 minutes, and form the mass into a ball. Leave it ex- 
posed to the air and examine it for the next half hour to see 
whether it hardens or sets in that time. Ordinarily it will 
take a longer time than this to stiffen, and it is only in extremely 
hot weather that it will be necessary to make this test. 

Packing Portland Cement. Portland cement may be ob- 
tained in paper bags, cloth sacks, or wooden barrels. The 
most convenient form for general use is either the cloth bag or 
the paper sack, which contains 1 cubic foot of cement, but 
ordinarily the cloth sacks should be used. These sacks can be 
returned to the dealer from whom the cement was purchased, 
and rebate obtained for them if they are kept dry and untorn. 

Storing Portland Cement. Portland cement is very suscep- 
tible to moisture and weather conditions, and must therefore 



MATERL\LS FOR PLAIN CONCRETE 5 

be stored in a dry place, such as a barn or shed. The cement 
will become lumpy or even form a solid mass when kept in a 
damp place, and when in this condition it should not be used. 
All lumps that do not crumble at the lightest blow should be 
thrown out. 

Cement stored in a building must not be placed on the bare 
ground. Make a platform that is at least 6 inches above the 
ground, and store the cement on this platform. If the cement 
is to be stored on a concrete floor it is advisable to cover the 
floor with planking upon which to place the cement. 




Fig. 2.— Fine Beach Sand, Greatly Magnified. (See page 5.) 

Definition of Fine Aggregate or Sand. The Fine Aggregate 
is any sand, or broken stone screenings, where the grains vary 
from % inch in diameter down to a very fine powder. 

Kinds of Sand. Sands may be roughly classified as fine, 
medium, and coarse. Specifications generally call for a clean 
sharp sand, but very seldom for a fine, medium, graded or coarse 
sand, which is in reality the most important consideration. 

A fine sand is like beach sand, with few particles over Y^ inch 
in diameter. See Fig. 2, page 5. A medium sand is one which 



6 PLAIN AND REINFORCED CONCRETE 

has practically no very fine or very coarse particles. A graded 
sand is sand having fine as well as coarse particles, neither 
predominating. A coarse sand is sand having very little fine 
material in it. 

The purpose for which the concrete is to be used and the 
kind of Coarse Aggregate determine somewhat the kind of Fine 
Aggregate to be used. See page 13. In concrete for floors it is 
best to use a very coarse Fine Aggregate, while in concrete 
which is to be water-tight graded sand may be used which has 




Fig 3.— Bank Sand, Greatly Magnified. (See page 6.) 

a large percentage of very fine material. Beach sand, on ac- 
count of its fineness, should not ordinarily be used. Fig. 3, 
page 6, shows a view (greatly magnified) of a good bank sand. 
The quality of Fine Aggregate cannot be determined by 
looking at it in the bank or pit, as the sand may contain vege- 
table matter which is difl&cult to detect without a special test. 
The ordinary plan of taking a little sand in the palm of one 
hand and rubbing it with the other to see whether it discolors 
the hand is practically of no value, and the other test of dropping 
sand in water to see the amount of turbidity is also impractica- 



MATERIALS FOR PLAIN CONCRETE 7 

ble. The discoloration of one's hand may be due to iron ore, 
and the turbidity may be due to clay, neither of which has any 
effect on the quality of the Fine Aggregate. It is the vegetable 
matter coating the grains, which cannot be detected in either 
of these ways, that is the harmful element. If the sand grains 
are coated with this very fine vegetable matter, even so small 
an amount as a fraction of one per cent, it may prevent the 
cement which surrounds the grains from setting, and thus 
make a worthless concrete. 

How to Test for Clean Sand. A rough test for clean sand is 
to pick up a double handful of moist sand from the bank and 
move the hands in a vertical position, keeping them about ^ 
inch apart, and allowing the sand to slip quickly between them. 
Repeat this operation a dozen times and then rub the hands 
lightly together so as to remove the fine grains of sand which 
adhere. Examine the hands very carefully to see whether or 
not a thin film of sticky matter adheres, and if so, the sand 
probably contains loam or vegetable matter. A further test 
of this is to scrape some of this matter from the fingers and take 
a little of it between the teeth. If it does not feel gritty, it is a 
fair indication that this matter is vegetable loam, and not 
fine powdered grains of sand. Do not use the sand, if it can 
be avoided, without further tests or washing. Sand may, 
however, contain roots or vegetable matter, but if these are 
in pieces and not in great quantities they will do no harm. 

A further test of questionable sand is to make up two 6-inch 
concrete cubes, using the questionable sand with the gravel or 
stone which is to be used in the structure. Keep one block 
in the air out-of-doors for a week and keep the other one in a 
fairly warm room for the same length of time. The specimens 
should be hard enough to remove from the molds in 24 hours. 
The first specimen may take a trifle longer if the weather is 
cold and damp. At the end of the week, test both blocks by 
hitting them with an ordinary hammer. If the hammer does 
not dent thena under light blows, such as would be used in 
driving tacks, and the blocks ring under the blows and are not 
broken under medium blows, the sand can be used with safety. 

Method of Using Dirty Sand. Sand is ordinarily washed on 
the mixing board. This is very unsatisfactory, as it generally 
simply transfers the dirt to the lower part of the pile instead 
of washing it out of the sand. If this method must be used, 
the board upon which the sand is washed should be raised from 
the ground so that the dirty water may flow away from the 



8 



PLAIN AND REINFORCED CONCRETE 



board and also away from the sand pile. A more satisfactory 
method of washing sand, provided it is not too fine, is to make 
an inclined washing trough, as shown in Fig. 4. This trough, 
which should be lined with tar paper so as to convey the water 
a safe distance away from the sand pile and washed sand, 
should be about 16 feet long and 30 inches high, set on a slope 
of about 5 feet in 12 feet. A screen is inserted in the bottom 




Fig. 4. — Screen and Trough for Washing Sand. (See page 8.) 



near the lower end, through which the dirty water will pass. 
For sand, a screen with 30 meshes to the linear inch is neces- 
sary to prevent the good particles from passing through. The 
sand is shoveled on to the upper end of the trough and washed 
down by a stream from a hose at the upper end. The flow of the 
water will wash the sand down the incline, and as the mixture 
passes over the screen, the dirty water will drain through, leav- 
ing the clean sand for use. By this arrangement, as shown in 



MATERIALS FOR PLAIN CONCRETE 9 

Fig. 4, page 8, the dirt which is washed out is caught in the 
water trough. 

Definition of Coarse Aggregate. Coarse Aggregate is broken 
stone or gravel whose grains vary from % inch diameter up to 
y^ inch and sometimes to 2^ inches, according to the class of 
work. 

In thin walls where reinforcement is used the largest size 
of coarse grains should be limited to ^ inch. For foundations 
or heavy walls over 12 inches thick, the maximum size of Coarse 
Aggregate can be 2^ inches. The reason for not using larger 
Coarse Aggregate in the thin walls is the danger of some of the 
stones getting caught and leaving a so-called '^pocket'' in the 
wall. This can be noticed at the time the concrete is being 
poured, and will not only make a bad appearance when the 
forms are removed but is liable to lessen the strength of the 
structure. 

In very large foundations it is even advisable to use some 
large cobble stones in the mass of the concrete, but in no case 
must these large cobble stones touch each other or extend 
through the wall. 

"Never use a soft sandstone, a soft limestone, shale or slate. 

Coarse Aggregate, like Fine Aggregate, may be found to be 
dirty, especially if the coarse aggregate is gravel. In some 
cases the gravel grains are coated with a fine powdery matter 
which may or may not be organic or vegetable matter. Bad 
results will follow if such gravel is used. 

For washing gravel of this character, the same kind of 
washing trough as shown in Fig. 2 may be used, except that the 
screen near the bottom of the trough should have openings 
about }^ inch square. 

Crushed Stone vs. Gravel for Coarse Aggregate. The ques- 
tion is often asked, what kind of Coarse Aggregate makes the 
strongest concrete. The general impression is that crushed 
stone is the stronger, but tests show that good clean gravel 
concrete is a trifle better than crushed stone concrete. The 
crushed stone, however, is more likely to be clean and has there- 
fore an advantage over the ordinary unwashed gravel. 

Natural Mixtures of Bank Sand and Gravel. The sand and 
gravel are generally found mixed in a pit, but they are not 
usually graded properly for concrete and should therefore be 
screened over a % inch screen. The expense of screening this 
material is not warranted on small work, and it therefore is 
better to use a slightly greater amount of cement than would 



10 PLAIN AND REINFORCED CONCRETE 

be used if the sand and gravel were screened and re-mixed. On 
any work of a large character it would probably more than 
pay for the extra labor required to screen the materials and 
re-mix them. For example, using even a very good gravel 
bank, a mixture of one part cement to four parts of natural 
gravel must be used, while if screened, one part of cement 
could be mixed with two parts of sand and four parts of screened 
gravel. In the above example one bag of cement more in every 
seven is required with the natural bank or unscreened gravel 
than with the material when screened and re-mixed in the 
right proportions. 




Fig. 5.— Pieces of Gravel Concrete. (See page 10.) 

Screenings from Crushed Stone. The screenings from crushed 
stone make an excellent substitute for sand, provided there 
has not been too much fine powder produced. The stone must 
not be of a soft or shelly character or contain a large percentage 
of mica. 

Water. Ordinarily it is not necessary to test the water for 
use in concrete. It should not be taken, however, from a pol- 
luted steam or pond, and generally sea water should not be used. 

The photograph of pieces of gravel concrete in Fig. 5, page 
10, shows the difference in the diameter of the stones used in 
the two cases. The sample on the left of the picture was 
taken from a wall footing while the concrete was green, thereby 
breaking the concrete around the stones. The sample of con- 
crete on the right of the picture was cut from a piece of con- 
crete taken from a column which had been cast for several 
months. 



MATERIALS FOR PLAIN CONCRETE 11 

REVIEW QUESTIONS 

CHAPTER I. 

1. What is plain concrete? 

2. What kind of cement should be used in plain concrete? 

3. What is a Portland cement? 

4. What is meant when it is stated that a cement has its initial set? 

5. What is meant when it is stated that a cement has its final set? 

6. What is an abnormal cement? 

7. What causes a cement to become abnormal? 

8. In extremely hot weather what test should be performed to see if the 

cement is O. K.? 

9. What determines whether a cement has a flash set? 

10. Should a cement having a flash set be thrown away? If so, why? If 

not, why? 

11. How should cement be stored when brought to a job? 

12. Why is a sand called a "Fine Aggregate''? 

13. How can a sand be tested to see whether it is a clean sand? 

14. How can a good strong concrete be made with a dirty sand? Is thi s 

good practice? 

15. Why is a gravel or broken stone called a "Coarse Aggregate'*? 

16. What kinds of Coarse Aggregate should never be used? 

17. What are the advantages, if any, of screening the sand from the gravel 

in a natural bank and remixing them properly? 

18. Can screenings from crushed stone be used in concrete? 

19. Is it necessary to test the water used in concrete? 



CHAPTER II 

PROPORTIONING MATERIALS 

Proportioning of materials is the proper adjusting of the 
relative quantities of Cement, Fine Aggregate, and Coarse 
Aggregate to each other. 

Concrete is defined in Chapter I as an artificial stone composed 
of broken stone or gravel varying from 2 inches down to % 
inch, and sand graded from % inch down to very fine powder, 
which is mixed very thoroughly and intimately with cement. 
The determination of the quantity of each of these materials 
is called "proportioning." 

Theory of Proportioning. The object of Proportioning the 
aggregate is to make the artificial stone or concrete of maxi- 
mum density, i. e., to have the fewest possible air spaces or 
"voids", as they are called, in the concrete. The method of 
determining the voids in a Coarse Aggregate is as follows : 

A Construct a box 12 inches square and 12 inches deep for 
measuring the aggregate. Weigh the measure empty. 

B Fill the measure even full with the Coarse Aggregate and 
weigh. Deducting the weight of the measure'gives the weight 
of the Coarse Aggregate, and as the measure holds exactly 
1 cubic foot of material, this weight is the weight of 1 cubic 
foot of the Coarse Aggregate. 

C The weight of a cubic foot of the Coarse Aggregate if 
it were a solid rock varies with the character of the material. 
The following weights of a solid cubic foot of rock of various 
materials can be used as shown: 



1 


Sandstone, 


150 


pounds 


per 


cubic foot. 


2 


Conglomerate, 


162 




it 




3 


Limestone, 


162 




li 




4 


Sand, 


165 




u 




5 


Gravel, 


165 




u 




6 


Granite, 


168 




li 




7 


Trap, 


180 




<( 





D The percentage of voids, therefore, is figured by dividing 
the weight of the Coarse Aggregate obtained in {B) by a cor- 
responding weight of the Coarse Aggregate, if solid, and multi- 



PROPORTIONING MATERIALS 13 

plying the product by 100. Then deduct this result from 100, 
which will given the percentage of voids. 

Illustration: — Assume the Coarse Aggregate to be crushed 
granite weighing 98J pounds in the measure as given in (B) 
above. Dividing 98J by 168 obtained from (C) gives 0.585. 
Multiplying 0.585 by 100 gives 58.5. Now deducting 58.5 
from 100 gives 41.5, or 41>^ per cent voids, which is the answer. 
In other w^ords, if the stone in the box could be made into a 
solid stone to fit the box, the box would be between 3^ and ^ 
empty. 

Try this same thing with gravel for a Coarse Aggregate 
instead of broken stone and note that the amount of voids in 
the gravel is different from that in the broken stone. In order 
to produce artificial stone these voids must be filled up. To 
do this, the Coarse Aggregate is mixed with finer material 
called the Fine Aggregate. Following out the same line of 
reasoning just used some voids will be found in the mixture, and 
whether this percentage is small or large depends upon the 
relation of the amount of Fine Aggregate added to the Coarse 
Aggregate. 

The question, then, is to know how much of each of the 
aggregates should be used in order that the least amount of 
cement may be used to bind these materials into a solid mass. 
This process of determining the quantity of each of the materials 
is called proportioning. 

Kinds of Mixes. There are four standard mixtures which 
are generally used, depending upon the purpose for which the 
concrete is to be used. These four mixtures may be classed as 
follows: 

A Very lean mix. 

B Lean mix. 

C Standard mix. 

D Rich mix. 

A very lean mix should only be used for large masses of con- 
crete, such as heavy walls, large foundations, and other con- 
struction not subject to vibration and for any place where the 
concrete is only to be used as mass work. For this mixture, 
use one barrel (4 bags) of Portland cement to 3 barrels (11.4 
cu. ft.) of loose sand to 6 barrels (22.8 cu. ft.) of loose broken 
stone or gravel. This is known as a 1:3:6 mix, 

A lean mix should be used in the base for sidewalks and 
floors, thin foundation and building walls, piers, abutments, 
and retaining walls. For this mixture, use one barrel (4 bags) 



14 PLAIN AND REINFORCED CONCRETE 

Portland cement to 2>^ barrels (9.5 cu. ft.) loose sand to 5 
barrels (19 cu. ft.) loose broken stone or gravel. This is known 
as a l:2j4:5 mix. 

A standard mix should be used for any work subject to vibra- 
tion, such as engine and machine foundations, tanks, sewers, and 
other water-tight work,and for reinforced concrete floors, beams, 
and columns. Use one barrel (4 bags) Portland cement to 
2 barrels (7.6 cu. ft.) of loose sand to 4 barrels (15.2 cu. ft.) 
broken stone or gravel. This mix is known as a 1:2:4 w^'^- 

A rich mix should be used for any concrete subject to high 
stresses or requiring exceptional water-tightness. Use one barrel 
Portland cement (4 bags) to lyi barrels (5.7 cu. ft.) of loose sand 
to 3 barrels (11.4 cu. ft.) broken stone or gravel. This is known 
as al:l}^:S mix. 

Amount of Cement vs. Strength of Concrete. The cementing 
or binding property of concrete is produced by the cement, as 
described in Chapter I. In the above paragraph, the subject 
of voids between the particles has been taken up. In order to 
bind the aggregates together, from the very finest powder to 
the coarse materials, it is necessary for the cement to coat all 
the surfaces of each grain and also fill the voids between the 
grains. The ideal condition is to use only enough cement to 
accomplish this, because the cement is the most costly part of 
the mixture. liF an excess of cement is used, part of the mass 
will be pure cement, and if too little cement is used, some of 
the voids will not be filled, and hence the mass mil not be 
united solidly together. 

Use of Very Fine Sand. If the mixture contains a large 
quantity of very fine sand there will be more surfaces for 
the cement to cover than if the particles were coarser. To 
illustrate: if a 1 inch cube, which has 6 square inches surface 
area, is broken into two pieces, the tw^o pieces will have 7 
square inches surface area. Following out this line of reas- 
oning by breaking it into smaller sections, the square inches 
of surface area increase with the number of breaks. Now, 
with a very fine sand this same condition exists, and is 
one of the reasons why a very fine sea sand is not recommended 
in Chapter I. 

Screening Bank Gravel. On any work of large character it 
will generally pay to screen the sand from the gravel in a natural 
bank, as there will be more voids in the unscreened gravel to be 
filled with cement than there would be if the material from 
the bank were screened and remixed in the correct proportions. 



PROPORTIONING MATERIALS 



15 



The cost of screening this material is much less than the cost 
' of the extra cement required if the material is not screened. 

After separating the bank gravel into the sand (material 
passing Y^ inch) and gravel (material larger than % inch), 
mix these in the proportions recommended on page 13. An 
illustration of the economy which may be effected is given by 
the following example. 

On one job a mix of one part of cement was required to 4 
parts of the natural bank gravel, which corresponded to a mix 
of 1 part cement to 2 parts of the screened sand below J^ 
inch to 4 parts of the screened gravel caught on a >^ inch screen. 
In this case a saving of one bag of cement in every seven was 
made by screening and re-mixing the materials in the right 
proportion. 






Q^O 



.n^ 




CO""' 



^cf^' 



Fig. 6. — Diagram Illustrating Measurement of Dry Materials and Amount of 
Concrete Produced by Mixing Them Together. (See page 15.) 

Volume of Concrete vs. Volume of Aggregate. A mistake 
which is often made by the inexperienced is to figure that one 
barrel of cement mixed with two barrels of sand and four 
barrels of stone will make seven barrels of concrete. They do 
not take into consideration the fact brought out in the be- 
ginning of this chapter that the sand fills the voids between 
the stones, while the cement fills the voids between the sancl, 
and consequently the total volume of the concrete will there- 
fore be only slightly in excess of the original volume of the 
stone. This is very clearly illustrated in Fig. 6, page 15. 



16 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS. 

CHAPTER II. 

1. What is meant by proportioning of materials? 

2. How can the voids in a Coarse Aggregate be determined? 

3. What is the object of proportioning of materials? 

4. What are the names of the different kinds of mixes? 

5. What is a very lean mix? 

6. What is a lean mix? 

7. What is a standard mix? 

8. What is a rich mix? 

9. Should a rich mix be used in all cases? 

10. Why is it not good practice to use a sea sand for concrete? 

11. Why should a bank gravel be screened? 



CHAPTER III 



MIXING CONCRETE 



Concrete may be mixed either by hand or by machine, de- 
pending upon the amount of concrete required. For all small 
work and for work even as large as the cellar walls of a house it 
is economical to mix concrete by hand. If a number of buildings 
are to be built it may be cheaper to buy a machine mixer. 

Concrete mixing is such a simple operation that it is often 
improperly done, with the result that instead of producing a 
first-class concrete an indifferent or even poor concrete results. 




Fig. 7.— Mixing Platform. (See page 17.) 

It is very poor economy to use more cement in a mix in order 
to allow for insufficient mixing or improper handling of the 
materials. The additional cement will be much more expen- 
sive than the extra time expended by laborers in securing a 
thorough mix. 

Specifications for Hand Mixing. Use a large water-tight 
mixing platform about 15 feet square (Fig. 7, page 17). If 
dressed lumber is used, the shovelling will be much easier. 



18 



PLAIN AND REINFORCED CONCRETE 



Measure very carefully the exact amount of sand needed and 
spread it evenly on the mixing platform. Use a measuring 
frame as shown in Fig. 10, page 20. 

Spread the required amount of cement evenly on the sand. 

Mix the dry sand and cement together by pushing the shovel 
along the platform under the pile. Lift the shovel about 2 feet 
ahead of the pile, then turn the shovel completely over, and with 
a spreading motion draw the shovel backward over the pile. 
Repeat this operation of turning and mixing three times. 

Wet the broken stone or gravel before using. 




Fig. 8. — Wood Tamper. (See page 19.) 



Measure the exact amount of broken stone or gravel needed, 
which will be twice the amount of sand used, and spread it 
evenly on a mixing platform, beside the mixture of sand and 
cement. Shovel the mixture of sand and cement on top of the 
layer of broken stone or gravel in a layer of nearly even thick- 
ness. 

Fill a definite number of pails of water and pour it evenly 
on top of the materials. 

Mix the stone, sand, cement, and water in the same manner 
as described for mixing the sand and cement. Repeat this 
operation three times. 

Tools and Apparatus. The number of tools required for a 
job varies with the size of the gang of men to be used. For a 



MIXING CONCRETE 



19 



small gang of two or three men the following schedule woiild 
be about right: — 

1 Mixing Platform about 15 ft. square. 

1 Measuring Frame. 

2 Iron Wheelbarrows. 

3 No. 3 Square Pointed Shovels. 
1 Garden Spade. 

1 Barrel for Water. 
3 Water Buckets. 

1 Sand Screen. 

2 Tampers. (Use a piece of 2 x 4 ft. joist). 

Sizes of Gangs. The number and arrangement of men used 
in a gang for mixing concrete by hand may differ widely, as 




Fig. 9 — Mixing Concrete by Hand. (See page 19.) 

they depend upon the method used in mixing, the location of 
the materials with reference to the mixing board, the distance 
of the mixing board from the place where the concrete is to 
be used, and upon the size and importance of the w^ork. What- 
ever method of mixing is used, the gang should be arranged so 
that each man will have a definite duty to perform. Fig. 9, 
page 19, shows a gang of men mixing concrete by hand. 

On small work two or three men may perform all the opera- 
tions, from getting the sand and gravel from the bank to the 
mixing and placing of the concrete in the forms; but on work 
like a cellar foundation it would be economical to have a gang 
of about six men. With a gang of six men, four men should be 



20 



PLAIN AND REINFORCED CONCRETE 







MIXING CONCRETE 



21 



assigned to the work of wheeling and mixing the concrete, 
while the other two men should look after the placing and 
ramming. 

Method of Measuring. The materials should always be 
measured for every batch of concrete, in one w^ay or another, 
to insure the same mixture in the whole structure. The ma- 
terials may be measured in a measuring frame (Fig. 10, page 20), 
in wheelbarrows or in headless barrels. If the materials are 
close to the mixing board, a measuring frame or a headless 
barrel can be used; while if the materials are at some distance 
from the mixing board they must be wheeled and therefore can 
be measured at the same time. 

The size of the measuring frame to use for different mixes 
of concrete is given in the table below. In each case the frame 
is made of such size that the mix will contain two bags of cement. 
The frame is filled once even full with the Coarse Aggregate of 
broken stone or gravel, and half full wdth the Fine Aggregate 
of sand to obtain the correct proportions. The loose board on 
top of the frame, as shown in Fig. 10, is made with the lower 
projecting part exactly one half the depth of the side of the 
frame and is moved over the top of the frame to even ofiE the 
sand when it is being measured, thus insuring the correct 
amount of Fine Aggregate. The upper side of this board 
should be used to level off the broken stone or gravel in the 
measuring frame, so as to have it just even full, 

SIZE OF MEASURING FRAME FOR DIFFERENT MIXES OF CONCRETE 
Use Only for a 2 Bag Batch 



Mix 



Number of Cu. 

Ft. of Concrete 

made from one 

2 Bag Batch 



Size of Measuring Frame 
Fill Frame once full when Measuring Broken 
Stone or Gravel 
Fill Frame one-half fall when Measuring Sand 



1 


:3 


:6 


12.8 Cu. 


Ft. 


\r 


Deep 


x2'- 


-r 


Wide X 4' 


-6" 


Long 


1 


:2i 


:5 


10.9 " 


** 


12" 


" 


x2' 


-r 


" x4' 


-6" 


(< 


I 


:2 


:4 


9.0 " 


** 


10" 


" 


x2' 


-4" 


" x4' 


-0" 


<( 


^ 


:1| 


:3 


7.0 " 




10" 




x2'- 


-0" 


" x3' 


-0" 





If the materials are wheeled and measured in wheelbarrows, 
great care should be used to see that the wheelbarrows are 
always filled with the same amount, thus insuring a uniform 
mix. To determine the number of wheelbarrows of sand and 
stone to use, and whether to fill them even full or with a crown, 



22 PLAIN AND REINFORCED CONCRETE 

make a box 12 inches square and 12 inches high in which to 
measure the materials. This frame will hold exactly one cubic 
foot of material. As an example of the method of using this 
one cubic foot measure assume that a 1:2^:5 mix is de- 
sired, and that the batch of concrete to be mixed at one 
time will contain two bags of cement. A bag of cement is 
always assumed to be a quarter of a barrel, and to contain one 
cubic foot of cement. If two bags of cement are used this is 
equivalent to 2 cubic feet of cement, and a 1:2^:5 mix will 
therefore require 5 cubic feet of loose sand, and 10 cubic feet 
of loose broken stone or gravel. Fill the one cubic foot measure 
with sand and dump the sand into a wheelbarrow. Fill the 
measure again and dump this sand in the same wheelbarrow. 
If the wheelbarrow is a standard iron barrow it will not be 
quite even full of sand. Fill the measure now only half-full of 
sand, and dump this also in the wheelbarrow and even oflf the 
sand. The barrow will be very nearly full. Note just how 
much the top of the sand is below a stick scraped across the 
top of barrow. This will now be your measure for the amount 
of sand to be used when measuring the Fine Aggregate. As 
just stated above, five cubic feet of sand are required for two 
bags of cement, and therefore two w^heelbarrow^s of sand should 
be used for the batch of concrete. In like manner, measure the 
broken stone or gravel with the cubic foot measure and dump 
into the wheelbarrow, and determine the number of wheel- 
barrows of broken stone or gravel which should be mixed with 
the cement and sand. 

Batch Sizes. ^^ Batch Sizes" is a term which has come to 
mean the number of bags of cement which are to be used in 
mixing up a batch of concrete. A two-bag batch means that 
two bags of cement will be used with whatever amount of sand 
and broken stone are needed to produce the desired mix. On 
very small work a one-bag batch is economical to use, but ordi- 
narily a two-bag batch is used. If too large a batch is mixed 
at one time, it is harder work to mix the batch thoroughly, 
it takes a very large mixing platform, and unless a gang of at 
least six men is used to mix the concrete there is danger of not 
getting a thorough mix, and in hot weather the concrete is 
liable to get its initial set. 

^^ Batch Sizes" is used also in connection with machine mix- 
ing of concrete. On very small work a machine might be con- 
structed as shown in Fig. 1 1 , page 23 . In a machine of this kind a 
one-bag batch would be mixed. The ordinary machines, as 



MIXING CONCRETE 



23 




24 PLAIN AND REINFORCED CONCRETE 

they are found on the market, vary from a two-bag batch to a 
four-bag batch. The two-bag batch machines are the ones 
most commonly used. 

There is another type of machine used for mixing the con- 
crete, called a *^ continuous mixer'' (Fig. 12, page 24). In this kind 
of machine the flow of concrete is continuous, as the name indi- 
cates, and consequently the machine is not known as a batch mixer. 

Consistency of Mix. The consistency of freshly mixed con- 
crete is its degree of plasticity or softness. The general terms 
used for the plasticity of freshly mixed concrete are ^^ quaking 
mix" and ^^dry mix." The quaking mix, or mix of jelly-like 
consistency, is used for ordinary work. 



WfY|/| 


1 


IPH^ 




\i 


Wm 






mBm 



Fig. 12. — Mixing Concrete with a "Continuous Mixer." (See page 24.) 

Always use a quaking mix except where the concrete must 
withstand severe compressive strain within a month after plac- 
ing. In the latter case use a dry mix. A concrete which re- 
quires tamping to flush water to the surface is known as a dry 
mix. When using a dry mix spread the concrete in layers not 
over 6 inches thick and ram the concrete thoroughly. 

When concrete is to be used in large masses like foundations, 
piers, thick walls, etc., its strength may be increased and the 
cost of the structure reduced by using large, clean stones which 
can be imbedded in the concrete. Great care must be taken 
that the stones are completely surrounded by concrete, do not 
touch one another, and are separated by a sufficient layer of 



MIXING CONCRETE 



25 



cement and sand. Never imbed large stones in concrete of dry 
consistency. 

Density. In Chapter II, under the heading of *^ Proportion- 
ing Materials/ ' it will be seen that one of the desirable features 
of good concrete is to have it of maximum density; that is, with 
the smallest percentage of voids. 

If two pieces of concrete are of the same size and of the same 
aggregate the heavier piece is the denser. 



26 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS 

CHAPTER III. 

1. What are the two ways of mixing concrete? 

2. Why should concrete be mixed very thoroughly? 

3. What is the proper method of mixing a batch of concrete by hand? 

4. Shoiild the materials be measured accurately? If so, how? 

5. How should the water be measured? 

6. What tools and apparatus should be used if two or three men are to do 

the mixing of the concrete? 

7. What is an economical number of men to use for mixing and placing 

concrete for a ceUar foundation? 

8. When should wheelbarrows be used to measure up materials? 

9. What is meant by a two-bag batch? 

10. What is the "consistency of a mix"? 

11. When should a "quaking mix" be used? 

12. When should a "dry mix" be used? 

13. How should a "dry mix" be placed? 

14. Is it proper to imbed large stones in a "dry mix" concrete? 

15. What is the relation between strength of concrete and density? 



CHAPTER IV 

PLACING OF CONCRETE 

Great care should be taken in placing the concrete to prevent 

the separation of the stones or coarse aggregate from the mortar. 

Ordinarily not enough attention is given to this part of the work, 

. with the result that a bad job will result, necessitating a lot 

of patching, which is both costly and unsatisfactory. 

Placing Concrete. Concrete may be placed by pailfuUs, 
wheelbarrows, buggies, cars, or by shutes. On all small work 
only the first two methods are used. Pails are used extensively 
on account of the saving in the making of runways which are 
necessary when wheelbarrows are used. The objection to the 
use of pails is the slowness of handling, but with a small gang 
of men it may prove economical. 

In transporting the concrete from the mixing platform to 
place it is essential to prevent the separation of the stones from 
the concrete. This is very noticeable in concrete which is 
mixed rather wet and wheeled quite a distance. The fine 
mortar will be on top and if the wheelbarrow man is not careful 
in dumping the concrete, he will pour off all the mortar and 
then have to shovel the coarse stone from the bottom of the 
wheelbarrow. This produces a very unevenly mixed concrete 
in the wall or structure. 

Placing Concrete in Forms. Concrete should be placed in 
forms in layers about 6 to 12 inches deep and tamped lightly 
with a rammer or tamper until the water flushes to the top of 
the concrete. Spade the concrete along the form by using a 
2 X 4-inch joist, as shown in Fig. 8, page 18. Thrust the spade 
between the concrete and the form, moving the handle up and 
down. This spading pushes the coarse stone away from the 
form so that when the form is removed the surface of the con- 
crete will show a uniform color with no stone pockets. These 
stone pockets are caused by the stones arching together in 
such a way as to prevent the finer materials from getting be- 
tween. By spading the concrete well these pockets will be 
broken up, thus saving the time and expense of plastering over 
cavities and smoothing the rough places. 

Placing Concrete Under Water. Concrete should never be 
placed under water if it can be avoided. The water washes 



28 PLAIN AND REINFORCED CONCRETE 

the cement away as the concrete sinks to the bottom, thus 
destroying the cementing or binding agent. If the concrete 
were to be deposited in a stream where there is a current, the 
result would be much worse. This conclusion is illustrated 
by a test made by the engineers constructing the Holyoke Dam. 
A small batch of concrete was mixed in the proportion of one 
part cement to two and one-quarter parts sand to five parts 
stone, and shoveled into a pail of water with a trowel. The 
surface hardened satisfactorily, and after several months the 
water was poured off and the material taken out. Instead of 
being concrete, three layers were found. On top was a thin 
layer of practically neat cement, then about 2 or 3 inches of 
mixed sand and cement in a porous mortar, then below this a 
mixture of sand and stone as separate and clean as before the 
concrete was mixed. This experiment shows that if concrete 
has to be placed under water it must be deposited in large 
masses and never by shovelfuls. 

On small work put the concrete in pails, place a board over 
the top of the pail and lower it carefully into the water to the 
bottom. Turn the pail upside down, carefully remove the 
board and slowly raise the pail, allowing the concrete to flow out. 
Great care must be used not to disturb the water in which the 
concrete is being placed nor to touch the green concrete. Con- 
crete must never be placed under water if there is any current, 
because the cement will be washed away, leaving only the sand 
and stone. 

Another method for depositing concrete under water is to 
pass the concrete slowly through a spout or tube which reaches 
to within a couple of inches of the bottom where the concrete 
is to be placed. The tube must be kept full and the concrete 
kept moving continuously and slowly through it. On large 
work specially designed buckets are used for depositing the 
concrete under water. 

Protecting Concrete in Hot Weather. Concrete which is 
exposed to the sun should be soaked with water each day for 
a week or two. This will allow the interior of the walls to dry 
uniformly with the exterior, and thus prevent scaling or crack- 
ing. 

Placing Concrete in Freezing Weather. Concrete work 
should be avoided as far as possible in freezing weather unless 
special precautions are taken to prevent the concrete from 
freezing before it has set. The alternate freezing and thawing 
of concrete before it has set is very likely to destroy the strength 



PLACING OF CONCRETE 29 

of the cement, or it may retard the set so long as to make it 
dangerous to remove the forms for a long time. 

The precautions to take in placing concrete in freezing 
weather are as follows: — 

a. Make tight and solid forms. 

b. Heat the stone and sand. 

c. Use hot w^ater. 

d. Use 2 pounds of salt to each bag of cement. (Dissolve 
the salt in the water.) As much as 3 pounds of salt to each 
bag of cement can be used if absolutely necessary in order to 
have the concrete get its initial set before freezing. 

e. Cover the top of the concrete with clean hay or straw, 
and if possible cover this with tar paper, old boards, or canvas. 



30 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS 

CHAPTER IV. 

1. Why should great care be taken in placing concrete? 

2. In what ways is concrete placed? 

3. Should concrete be dumped directly into a wall from a wheelbarrow in 

which the concrete of sloppy consistency has been transported 
a long way? Why? 

4. What is meant by spading the concrete? 

5. Why is the concrete spaded? 

6. What is the proper method for depositing concrete under water? 

7. How should freshly laid concrete be protected in hot weather? Why? 

8. What precautions must be taken in order to place concrete in freezing 

weather? 



CHAPTER V. 

REINFORCED CONCRETE 

Definition of Reinforced Concrete. Reinforced concrete is 
plain concrete in which steel or iron is imbedded. 

Use of Steel in Concrete. There are two reasons for the use 
of steel in concrete: (1) It increases the strength of the Plain 
Concrete. (2) It prevents cracks due to shrinkage. 




Fig. 13. — Concrete Beam with Steel Reinforcement in Bottom of Beam. (See page 31.) 

Concrete, as already stated, is an artificial stone having the 
same strength as natural stone. It is generally known that a 
tremendous load can be placed on a firmly embedded stone with 
very little fear of its being crushed, that is, the stone is very 
strong in ''compression." Now take a stone like a window 
sill and support it only at the ends. This stone, unless it is 



LLU 



loQse Block 



/Thain-^ 



Rein forcement^ 
' E 

Fig. 14. — Reinforced Concrete Beam Showmg the Reinforcing Steel Taking Tension 
and the Concrete Taking Compression. (See page 31.) 

supported at the center, can be broken with a comparatively 
light load because the stone is weak in ''tension." If this win- 
dow sill is made of reinforced concrete with some steel bars 
cast in the bottom, it will be unlike the natural stone in that it 
will be able to take a very heavy load. The reinforced concrete 
is then strong in "tension" as well as "compression.'' To 
illustrate how a reinforced concrete beam is strong in tension 
as well as compression refer to Figs. 13 and 14, page 31. Fig. 13 



32 



PLAIN AND REINFORCED CONCRETE 



shows a reinforced concrete beam with the steel in the bot- 
tom of the beam. When the beam is loaded it has a tendency 
to bend, as shown by the dotted lines. The upper part of the 
beam is in compression which is fully resisted by the concrete as 




Fig. 15.— Reinforced Concrete Beam with Steel in the Top of Beam, Badly 
Cracked Under a Very Small Load. (See page 34.) 




Fig. 16.— Reinforced Concrete Beam with Steel in the Bottom of Beam, Sup- 
porting a Very Large Load without Cracking. (See page 34.) 

it is strong in compression. The lower part of the beam is in 
tension which is fully resisted by the steel. Fig. 14 shows how 
reinforced concrete is strong in compression at the top of the 
beam and strong in tension at the bottom of the beam. The 
beam in Fig. 14 is the same as in Fig. 13, excepting that the cen- 



REINFORCED CONCRETE 



33 




Fig. 17. — Spiral Column Reinforcement. (See page 34.) 



34 PLAIN AND REINFORCED CONCRETE 

ter section of the beam is cut away and is replaced by a loose 
block at the top and a chain attached to the steel in the bottom. 
With the load on the top of the beam the block is firmly wedged 
in between the cut beam because it is in compression, and the 
chain is taut because it is in tension. 

Fig. 15, page 32 shows a concrete beam in which a steel bar has 
been cast in the top of the beam and is badly cracked under a 
very small load. The steel in this case is in the wrong place to 
take the tension and so is of little value in increasing the 
strength of the beam. In Fig. 16, page 32, this same beam has 
been reversed so that the steel is in the bottom of the beam 
and is able to support a very large load. 

An invariable rule in placing steel is to insert it in the con- 
crete where the pull will come. In a beam or slab place it 
close to the bottom. In a wall, to withstand earth pressure, it 
must be in the face nearest to the earth. 

Concrete, like natural stone, is brittle, but by imbedding 
steel or other reinforcement in the concrete the concrete and 
the steel act together. This makes it capable of withstanding 
jars, and is why reinforced concrete is being used for engine 
foundations and bridges. 

Concrete expands and contracts with heat and cold. Con- 
crete shrinks also when it sets, so that in any large mass- 
work, like walls for buildings, it is necessary to place steel bars 
in the concrete. If no steel bars are used, large, irregular, un- 
sightly cracks will appear in the wall. If steel is used these 
cracks will be prevented, provided the steel bars are of smaU 
diameter and are distributed evenly, both vertically and hori- 
zontally. In large structures expansion joints must be left 
at regular intervals to take care of the expansion and contrac- 
tion of the concrete. 

Kinds of Reinforcing Steel. There are four kinds of reinforc- 
ing steel which are generally used: 

(1) Round Bars, which may be either plain or deformed. 

(See Fig. 17, page 33.) 

(2) Square Bars, which may be plain, twisted or de- 

formed. (See Fig. 18, page 35.) 

(3) Expanded Metal. 

(4) Miscellaneous, such as chicken wire, old steel rails, 

or old iron pipe. 

Round bars, square twisted bars, or bars with special sur- 
faces designed to prevent the bars pulling out from the concrete, 
are used in most of the important work in reinforced concrete. 



REINFORCED CONCRETE 



35 




36 PLAIN AND REINFORCED CONCRETE 

For slabs, metal fabrics, like expanded metal or woven wire, 
are frequently used instead of bars. If the amount of steel re- 
quired in any structure is large it will pay to figure, or have 
figured, the exact size and spacing of the bars. 

Never use steel of any kind that is coated wdth scales of rust 
or dirt which will prevent the bonding of the concrete with the 
steel. If the steel is coated, brush it off thoroughly with wire 
brushes. 

Corrosion of Metal Reinforcement. Tests and experience 
indicate that steel which is sufficiently imbedded in good, 
dense concrete is w^ell protected against corrosion, whether 
located above or below w^ater level. For protection against 
corrosion the steel must be covered by at least one inch thick- 
ness of concrete. If the concrete is porous, so as to be readily 
permeable by w^ater, as when the concrete is laid wdth a very 
dry consistency, the metal may rust away entirely on account 
of the presence of moisture and air. 

Bond Between Steel and Concrete. The cement which binds 
the sand and gravel together also binds the steel to the concrete. 
In order to pull a straight steel bar from a piece of concrete it 
is necessary to break the so-called bond. If the bar is tw^isted 
or deformed when placed in the concrete there is not only the 
bond such as exists on the surface of the straight bar, but there is 
a mechanical bond also, due to the twisted or deformed portions 
of the bar. This is the reason why a twisted or deformed bar 
is said to have a greater ^^bond stress" than a plain bar. 

Quality of Reinforced Steel. There are three kinds of rein- 
forcing steel: 

(1) Ordinary Iron Bars. (The so-called iron bars, as 

found in the market are almost always steel.) 

(2) Mild Carbon Steel. (Reinforced steel, which is 

generally used, is of this quality.) 

(3) High Carbon Steel. (High Carbon Steel should 

generally be avoided.) 

Fireproofing. The actual fire tests of reinforced concrete 
have been limited, but experience indicates that concrete, on 
account of its low rate of heat conductivity, and the fact that 
it is incombustible, may be safely used for fireproofing purposes. 

The thickness of concrete for fire protection depends upon 
the duration of the fire that is likely to occur in a structure. In 
beams the reinforcement should be protected by at least 1}4 
inches of concrete; in slabs by 1 inch; and in columns and 
girders by 2 inches. 



REINFORCED CONCRETE 



37 



Joints. For concrete construction it is desirable to cast the 
entire structure at one operation, but as this is not always possi- 
ble, especially in large structures, it is necessary to stop the work 
at some convenient point. This point should be selected so 
that the resulting joint may have the least possible effect on 
the strength of the structure. Ordinarily joints can be made at 
or near the center of the span. In building construction the 
joint should be made at the top of a column, that is the column 
should be poured first, which will allow for the shrinkage of 
the concrete upon setting, and then the beam, girder and slab 
should be poured at one operation. 

Precautions. The failure of reinforced concrete structures 
is usually due to any one or a combination of the following 
causes : 

(1) Poor Materials. 

(2) Faulty Execution. 

(3) Premature Removal of the Forms. 

(4) Defective Design. 




Fig. 19.— "Self-Centering" Reinforcement for Use in Walls, Roots and Floors. 



38 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS 

CHAPTER V. 

1. What is Reinforced Concrete? 

2. Why is steel used in concrete? 

3. What stresses does the steel in the concrete take? 

4. What stresses does the concrete take? 

5. Why is a concrete beam having the steel in the top of the beam prac- 

tically no stronger than a concrete beam without any steel? 

6. In what part of a concrete beam or slab should the steel be placed? 

7. In a retaining wall where should the steel be placed? 

8. Does concrete swell and shrink with heat and cold? If so, what 

should be done? 

9. What kind of steel is used for Reinforcing Steel? 

10. In what way can metal reinforcement in concrete be kept from cor- 

roding? 

11. Is there any bond between the steel and the concrete? 

12. What kind of steel is best to use for reinforcing? 

13. What thickness of concrete should cover the reinforcing steel in order 

to protect it properly from fire? 

14. Why is it necessary to make joints in concrete work? 

15. What are the fovir causes which cause reinforced concrete structures to 

fail? 



CHAPTER VI 

FORMS FOR CONCRETE 

Concrete when first mixed is a semi-fluid, and in order to 
cast it in some definite form a mould of some kind must be used 
to hold the concrete until it has ^^set up." This mould is gen- 
erally called diform. After the concrete has set up and hardened 
properly, the form can be removed, thus exposing the con- 
crete in the form or shape desired. A form is made by nailing 
sheathing boards to cleats or studs in such a way that the form 
can be removed as one unit or be taken apart easily after the 
concrete has set up. 

The forms are the most costly part of concrete work. The 
reasons for this high cost are the high price of suitable lumber, 
the expense of making, placing, removing and remaking them. 
Unless the forms can be used a number of times the cost might 
be so high that it would be found cheaper to construct the 
structure of some other material. 

The difl&cult part in the construction of the forms is to design 
them so that the resulting mass of concrete will be cast in 
the shape desired, and so that the forms can be readily removed. 
One of the difficulties in designing the form work is to so design 
it, that the Vv^ood upon becoming wet, and consequently swell- 
ing, does not break the concrete after it has set, or wedge it- 
self into the concrete in such a w^ay that it cannot be readily 
removed without the danger of cracking or even destroying 
the concrete entirely. The forms must be so constructed that 
they can be removed in sections, if they are to be used over 
again; and in case they are only to be used once it is desirable 
to have the sheathing boards in as large pieces as possible so 
that they can be used for other purposes. 

Design of Forms. The concrete forms must be designed so as 
to build them either 

A In Place. 

B In Sections or Units. 

Forms are generally built in place when they are to be used 
only once or twice. This occurs in cellar wall forms, especially 
where the outHne of the building has many offsets, bay windows, 
etc. Also small structures should be built in place, like water- 
troughs, gate-posts, engine foundations, etc. The advantage 



40 PLAIN AND REINFORCED CONCRETE 

of this is that it is not necessary to cut up the lumber nearly 
as much as if the forms were made in sections, thus making 
this lumber available for use somewhere else. 

If the cellar is regular, that is, practically rectangular, with 
only one bay at the side, it will probably be more economical 
to build sectional forms, so as to use only enough lumber for 
four or eight sections. These forms can be used over and 
over again until the whole cellar is finished. 

For example: — Take a basement 20 ft. x 30 ft. and 9 ft. deep. 
If the forms are constructed in place, 2 inch x 4 inch studs 10 
ft. long placed vertically every 24 inches both on the inside 
and outside of the wall would be used. Across these studs 
sheathing would be placed horizontally. The joint between the 
sheathing boards must come at a vertical stud, or otherwise 
the weight of the concrete will spring the end of the board 
between the studs. See Figure 20, page 40, showing the correct 
and incorrect method of butting sheathing boards. 



r-y^ i 



Rijht Method Wrong Method 

Fig. 20. — Right and Wrong Method of Butting Sheathing Boards. (See page 40.) 

Always place the biggest dimension of the stud perpendicu- 
lar to the wall or line of stress. 

The amount of lumber in the above example is as follows: — 

Sill 2" X 6'' to rest studs on = 200 ft. B.M. 

Studs 2'' X 4" — 24'' c. c. — 670 ft. B.M. 

Sheathing 1'' t. & g. = 1,800 ft. B.M. 



2,670 ft. B.M. 
at $30 per M = $80.00 
If sectional forms are used, use two sets of sections 3 ft. high 
and lumber will cost about $40.00, which will make a saving 
of approximately $40.00 on lumber. 

Size of Form Board. Lumber used for form work varies in 
thickness, width, and length. The variations in the thickness 
and width of the lumber will first be considered, leaving the 
variations in length to be taken up later under the discussion 
of grade of lumber to use in form work. 

Thickness of Form Boards. One of the first questions 
which must be answered is in regard to the proper thickness of 
lumber to use. The points to be considered are: — 1st, the 
thinner the lumber, the more often it will have to be supported 



FORMS FOR CONCRETE 41 

by studs; 2nd, the thinner the stock, the greater the breakage; 
3rd, the thinner the stock, the easier it is to handle (not so much 
weight); and 4th, the thin stock costs less than thick stock. 
In order to determine the most economical thickness to get it 
will be necessary to know the spacing of the vertical 2" x 4" 
studs for the different thicknesses of sheathing. 

Space 2" x 4" studs not over 2 ft. apart for 1" sheathing. 

Lumber used ior form work comes 1", IJ", 1^", If'', and 2" 
thick. This is the thickness which is paid for and is the thickness 
of the lumber in the rough, commonly called undressed lumber. 
In order to use lumber for form work it must be dressed, at 
least on the face side which comes in contact with the concrete. 
The reason for this will be taken up later. Dressing the lumber 
takes from \" to \" off of the thickness. Thus 1" boards will 
actually measure about I", and 1\" lumber will measure 1|", 

Width of Form Boards. The second consideration is the 
width of the sheathing. Ordinarily the wider the board the 
fewer the boards which will have to be nailed to the studs, and 
the stronger the form. A 10" board is generally stronger than 
two 5" boards, especially if the 5" boards have no tongue and 
groove. On the other hand, the wide boards warp more than 
narrow ones and also crack and shrink more. It is not generally 
advisable to use stock over 8" wide. 

The width of the lumber used for form work ranges from 5" 
to 10". In planing or dressing the lumber the width is reduced 
from I" to \% as explained under heading of ^^ Thickness of 
Form Boards." 

If the lumber is tongued and grooved the actual available 
width of the lumber is reduced by about f ". A 6" board will 
therefore measure about 5|", an 8" board about TJ". 

Length of Eorm Boards. Lumber generally comes in lengths 
from 6 ft. to 24 ft. by even feet, i. e., in 6 ft., 8 ft., 10 ft., etc. 
and up to 24 ft. The reason for this is that the lumber men 
consider a 12 ft. length to mean any board having an actual 
length of from 12 ft. to 13 ft. 11". In a few sections of the 
country lumber is sold in odd lengths also. A car of lumber, 
if ordered ^^ random lengths,'' will contain boards as short as 
6 ft. and only a few boards w411 run over 18 ft. 

Green vs. Seasoned Lumber. Green lumber, i. e, lumber 
containing the sap, should be used for form work because it 
will not swell and warp like seasoned lumber when the wet 
concrete is brought into contact with it. Never use seasoned 



42 PLAIN AND REINFORCED CONCRETE 

lumber unless it is water soaked. Of course, if green timber 
is bought for form work and allowed to remain in the hot sun 
either before or after it is made up into forms it will shrink^ 
but with a little care no great trouble should be experienced. 
In extremely hot weather the lumber should be wet often and 
thoroughly. If, after forms are made up, they shrink by ex- 
posure to the sun, they should be thoroughly soaked with a hose 
before the concrete is placed, which will make them swell and 
become tight again. 

Kinds of Lumber. White pine is the best wood to use for 
forms, but in many sections of the country its high price makes 
it prohibitive. Other woods which are used to a great extent 
are fir, yellow pine, and spruce. 

Square Edge vs. Tongue and Groove. Ordinarily square 
edge boards can be obtained easier than tongue and groove. 
It is more economical to use square edge boards than other 
kinds when the amount of surface area which 1000 ft. B. M. will 
cover is considered. It has been previously stated that the width 
of a tongue and groove board is ^" less than the width paid for. 
The square edge board only loses between \" and \" in dressing. 
Therefore 1000 ft. B. M. of square edge boards 1" thick will 
cover about 960 square feet of concrete surface, while 1000 ft. 
B. M. of the tongue and groove boards mil only cover about 860 
square feet, or approximately 100 square feet less of surface 
area per 1000 ft. B. M. 

Any form which is made and left to dry before using will 
shrink, leaving cracks between the boards which will fill up 
with concrete when it is poured and leave fins on the surface 
of the concrete. These fins make a bad looking job and in 
order to remove the forms they must be broken off, and in so 
doing the forms are racked considerably so that the nails come 
out and the life of the forms are very much lessened. Always 
wet the forms well with a hose before pouring the concrete. 

Tongue and groove lumber has an advantage over square 
edge lumber for form work in that it makes a stronger form 
because of the connection of the tongue in the groove. A form 
made of tongue and groove boards approximates in strength 
a form made of one wide board. 

Planed vs. Rough Lumber. All lumber for form work should 
be planed on the face coming in contact with the concrete. If 
the lumber is rough, the fibers of the boards will get stuck in 
the concrete, and when the form is removed the caught fiber 
will pull off a very thin film of concrete.* Some of the fibers, of 



FORMS FOR CONCRETE 43 

course, will tear off and remain stuck in the concrete, making 
the surface of the concrete very rough also, which looks badly. 
Rough or undressed lumber should never be used for lumber 
which comes in contact with the concrete but should be used for 
cleats, studs, and posts but it must be free from large knots or 
shakes. 

Dimensions of Cleats, Studs, and Posts. The dimensions of 
cleats, studs and posts are dependent upon the strength needed. 
It would be useless to use 4" x 4" cleats on a form where 1" 
lumber is used, for it would be impossible to place the amount 
of strain upon it that a 4" x 4" cleat could take without break- 
ing the sheathing between the cleats, therefore a 2" x 3" cleat 
would serve the purpose just as well as a 4" x 4" cleat. The 
ordinary sizes of cleats, studs, and posts are as follows: — 
A Sizes of cleats 

a 2'' X 3" stock 

b 2"x4" '' 

c 3"x4'^ '' 
B Sizes of studs and posts 

a 2" x 4" stock 

b 2 pieces of 2" x 4" stock nailed together to form 3.4:'' x 4:" 

c 3" X 4'' stock 

d 4''x4" " 

Grade of Lumber for Form Work. The grade of lumber to 
buy for form work depends upon: (a) the kind of a surface 
wanted; (b) the number of times the forms will be used; (c) the 
amount of strain to be put on them; (d) whether the lumber 
will be used for some other purpose after the concrete is poured; 
(e) the market value of lumber, and (/) whether it can be 
obtained immediately in the grade desired. 

Ordinarily second grade lumber is good enough for concrete 
forms. It ought to be sorted over when it arrives, and the broken 
or knotty pieces put into separate piles. Knots, if they are 
tight and not so large as to extend across the width of the 
stick, or if there are no series of knots across the stick so as to 
weaken it too much, are not particularly objectionable. If 
a knot does fall out of a board after the form is made up the 
hole can be covered over with a piece of tin or other metal. In 
making up the form, however, do not use boards from which the 
knots have fallen out but have the hole cut out, thus making 
a much stronger form. If the knot hole is small, do not waste 
the lumber by cutting out the hole. Never use lumber with 
knots for cleats or clamps. 



44 PLAIN AND REINFORCED CONCRETE 

Another consideration which has a bearing on the grade of 
lumber to select is whether or not the lumber can be obtained 
in the length which will work up most economically. For in- 
stance, if the length of a beam member is 12 ft., then 12 ft. stock 
should be used and not 14 or 16 ft. stock, on account of the waste 
at the end, or 10 ft. stock on account of the time and expense of 
splicing the boards. 

Oiling Surfaces of Forms Coming in Contact with the Con- 
crete. Concrete will adhere to wood or any other material 
unless special precautions are taken. Wood, which is the 
commonest material used for form work, is very porous, and if 
any wet material is placed against it, it will absorb the water 
from the material. Besides this, the surface of the boards, 
although they have been planed and feel smooth, have very 
fine strands protruding from the surface. The creamy cement 
particles run into the minute grooves between the protruding 
strands of wood fibre and harden. In order to break the form 
loose from the concrete these projecting fibres of wood must 
be torn off or the cement in the grooves between the fibres must 
be broken. The wood fibre is very strong, and in forcing off 
the form the surface of the concrete is found to be very rough 
and the form will have large scales of concrete adhering to it. 

One way to partly overcome this difi&culty is to wet the form 
thoroughly before the concrete is placed. This will prevent the 
form absorbing water from the concrete. The water used in 
wetting the form will fill up the spaces and roughnesses of the 
form and so prevent to a certain extent the creamy cement 
particles from getting into the grooves between the wood fibre, 
and thus the form will break away with comparative ease 
and without pulling off any large scabs from the surface of the 
concrete. 

In construction work the best practice is to wet down the 
forms with a hose thoroughly and then apply a thin coat of 
oil. In some cases the oil is applied directly to the form without 
wetting. Forms which have been oiled will come off easily and 
the concrete surfaces will be smooth. The reason for this is 
that the oil will not mix with the water from the concrete, and 
the oil will prevent cement from crystallizing; therefore the 
creamy cement particles in contact with the oiled form lose 
their cement or gluing property, and thus the form has no 
connection with the concrete. 

Kinds of Oil. The materials commonly used for oiHng the 
forms are soap, linseed oil, or a mixture of lard, kerosene and 
crude oil. 



FORMS FOR CONCRETE 45 

Method of Supporting Forms. Concrete when placed in 
forms produces a tremendous pressure. It is good practice to 
assume that concrete produces the same amount of pressure 
as water, and in about the same way. This assumption is on 
the safe side and justifiable. 

Concrete weighs approximately 150 lb. per cu. ft., and it 
not only produces a pressure after it is in the form, but it is 
generally dumped into the form in large quantities. To with- 
stand the impact of dumping, and the ramming and puddling 
it is necessary to make the forms strong and to brace them well. 
Walls along the ground can be braced by pieces of wood 
wedged against the form, but after the structure gets over 4 ft. 
high, it would take too much lumber to use this method and 
the two forms must therefore be tied in position by wdres or 
bolts. Fig. 25, page 53, shows a form braced from the sides, 
a good form for low cellar wall foundations. Fig. 26, page 55, 
shows a method of wiring two forms together, as well as a 
method of holding the forms by means of bolts. The spacers, 
shown in Figs. 25 and 26, are only placed between the forms 
to hold them the proper distance apart so that the forms can 
be drawn up tightly by the wires or bolts. 

Removing Forms. Great care should be taken that the forms 
are not removed too soon, for when the forms are loosened the 
concrete must support its own weight. In a large wall the 
material at the bottom of the wall supports the weight above 
it so that if the concrete is not hard it wdll be crushed out by 
the weight above. The length of time the forms should be left 
in place varies with conditions, and no definite laws can be for- 
mulated. The length of time for leaving in forms, as given below, 
are tentative, and a good deal of judgment must be used in 
following them. 
A With no pressure leave forms in place from one half to 

two days. 
B On small work, like drains and tile, leave forms in place 
about four hours, or until concrete will bear pressure of 
the thumb without appreciable indentation. 
C For large or important walls leave forms in from 1 to 3 

days. 
D If any earth or water pressure is present do not remove 

forms in less than 3 or 4 weeks. 
E Slab forms can generally be removed in about 1 week, 
but be sure and leave in supporting posts under girders, 
beams and slab for 1 month. 



46 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS 

CHAPTER VI. 

1. Why are the forms the most costly part of concrete work? 

2. Why is it difficult to design the forms? 

3. What are the two ways forms can be built? 

4. What thickness of lumber are used for forms? 

5. What widths of limiber are used for forms? 

6. What lengths of lumber are used for forms? 

7. What is the best kind of wood to use for forms? Is it used extensively? 

Why? 

8. What is meant by square edge boards? 

9. What is meant by tongue and groove boards? 

10. Whsit kinds of boards make the strongest forms? Why? 

11. What is meant by undressed lumber? By dressed lumber? 

12. Where can undressed lumber be used? 

13. What size lumber can be used for cleats? For studs and posts? 

14. Why must the lumber coming in contact with the concrete be planed? 

15. What grade of lumber is generally bought for forms? 

16. What determines the grade of lumber to be used for forms? 

17. Can lumber be used which has knots in it? When? How? 

18. Should lumber which has knots be used for cleats or clamps? 

19. Why are forms oiled? 

20. What kinds of oils are used on forms? 

21. How are forms held from spreading? 

22. How long after pouring the concrete should the forms be removed? 



CHAPTER VII 



SURFACE FINISH 



Surface finish of concrete is generally for the purpose of im- 
proving the appearance of the exposed surfaces of the concrete, 
but may be done in some cases to make the concrete more 
water tight. The surface of the concrete in contact with the 
forms will show every minute irregularity of the boards as well 
as the cracks between them. These surfaces can be finished 
so as to take off these marks and at the same time take off the 
skin of cement on the surface of the concrete, thus exposing the 
sand and stone and leave an even but slightly rough surface 
which will be pleasing in appearance. 



\ 




Fig. 21. — Artistic Effect Produced by Tooling Sur- 
face of Concrete. (See page 49.) 

Rubbing Surface with Brick. The best method of obtaining 
a good outside finish is to rub off the concrete surface while 
the concrete is green. This is done by removing the forms as 
soon as the concrete is set, which varies from 24 to 48 hours. 
Wet the concrete surface thoroughly and rub it with a brick, 
board, or carborundum block. This will remove the roughness 
on the wall and also the skin of cement, thus exposing the ag- 
gregate. The surface of the concrete while rough in appear- 
ance will be uniform in color and pleasing in effect. 

Picking Surface. The surface of the concrete to be picked 
must be much harder than if surface is to be rubbed with brick 
or board or the tool will loosen the stones If the surface is too 
hard it will take unnecessary labor to pick it. A stone cutter's 



48 



PLAIN AND REINFORCED CONCRETE 




SURFACE FINISH 



49 



bush hammer can be used for this picking, or a tool can be made 
having a toothed edge. 

Fig. 21, page 47, shows how artistic effects can be produced 
by tooling the surfaces of the concrete. Fig. 22, page 48, 
shows a photograph of a section of a wall which was finished 
off by tooling the surface with the tools shown in Fig. 24, page 
51. Fig. 23, page 49, show^s how fluted columns can be finished. 

Plastering Surface. Plastering on exterior surfaces requires 
great care and skill to prevent cracking and peeling. The 
forms in which the concrete is laid must be wet instead of oiled. 
Roughen the surface, either when the concrete is green, by 
rubbing off the cement, or by picking the hardened surface with 
an old hatchet or a stone axe. Wet thoroughly and apply as 




Fig, 23. — Fluted Columns Finished Off by Tooling. (See page 49.) 



thin a layer as possible, about 1-16 inch thick is best, of mortar, 
one part Portland Cement and one part fine, but very clean 
sand. For thick layers, pick and wet the surface, then brush 
on a thin coat of pure cement grout on a small part of the sur- 
face, and before this has begun to stiffen apply the plaster. 

Washing Surface with Pure Cement Mortar. Exterior sur- 
faces of concrete should not be coated with a pure cement 
mortar as the cement will check with fine hair cracks because 
of the rapid drying out of the mortar. However, for the interior 
of a tank which will be kept wet while in use, a coat of neat 
cement mortar may serve to make the concrete more water- 
tight. The cement mortar should be put on just as soon as 
the forms are removed, and these forms should be taken off as 
early as possible. On small pieces of concrete, like a small 



50 PLAIN AND REINFORCED CONCRETE 

trough, the inner form may be removed within three or four 
hours, and the wash appHed immediately. Leave the outside 
forms, however, until the concrete is hard. Wet the inside 
surface thoroughly and apply the pure cement mortar with a 
brush or a trowel. 



SURFACE FINISH 



51 




Fig. 24 — Tools Used for Finishing Off a Concrete Wall. (See page 49.) 



52 PLAIN AND REINFORCED CONCRETE 

REVIEW QUESTIONS 

CHAPTER VII. 

1. What are the two objects of surface finishing? 

2. What are the different methods used in surface finishing? 

3. Can a water trough be made water-tight by surface finishing? How? 

4. If the surface of the walls are to be plastered, how should the forms be 

treated before the concrete is placed? 

5. Is it practical to finish a surface by applying a neat cement mortar to 

the wall? 



CHAPTER VIII 



CONCRETE DESIGN 



Instructions, tables, and designs will be given of the simpler 
structures which may be built without the assistance of an en- 
gineer to figure and design them. 

Walls. Concrete walls should be made using 1 part cement 
to 2% parts sand to 5 parts broken stone or gravel. Carry the 




Fig. 25.— Cellar Wall Form. (See page 55.) 



footings of the walls down below frost line. In general, carry 
the footings down to about 4 feet below the ground level in the 
Northern and Middle States, about 3 feet in the Southern 
States, and in very mild climates where no heavy frosts occur 
2 feet will be sufficient. Spread the footings under the walls, 
that is, make them protrude on either side of the wall from 
4 to 6 inches. It is not absolutely necessary to have a spread 
footing in every case but it is good practice to do so because it 



54 



PLAIN AND REINFORCED CONCRETE 



distributes the load coming upon the wall over a larger ground 
area, which decreases the unit load which the ground must 
carry. For an example, take a wall 12 inches thick and assume 
that it carries a load of one ton to a running foot which means 
that the earth below the wall supports a load of 1 ton per 
square foot. If the wall had a spread footing protruding 6 
inches on each side of the wall, making it 24 inches across bot- 
tom of footing, then the ground would only support a load of >4 
ton per square foot. 

MATERIALS FOR CELLAR WALLS 

Proportions 1 : 2>^ : 5 



Foundations should generally be 4 feet below ground level. 
Earth must not be filled in against back of wall until 3 or 4 weeks* 
time after placing concrete. 

Quantities are for each 10 lineal feet of wall. 



o 


CO 

1 = 


Cement 


Sand 


Gravel 




If 




8 


O 03 


Ft. 


In. 


Bags 


55 


6 

8 

10 


6 

9 

12 


64 1 
13 


9 


15i 
304 
504 


5i 
11 
18 


304 
61 
101 



The average load for a one horse dump cart is 15 cubic feet (approx.) 

Cellar and Basement Walls. Cellar and basement walls must 
necessarily withstand the earth pressure that comes upon them, 
and for this reason are made from* 6 inches to 12 inches in 
thickness depending upon the height of the wall. Table on 
page 54 gives the thickness of walls required for different 
heights of cellars and also the quantities of materials required 
to construct 10 lineal feet of wall. The thicknesses of cellar 
and basement walls are less than for retaining walls" as the 
floor timbers brace the building walls and the weight of the 
super-structure also helps to strengthen it. 

When excavating the cellar where the ground will not stand 
with a vertical face excavate at least 18 to 24 inches outside 
of the building line to allow room for the form work so that the 
outside form can be readily removed and also to prtvent any 



CONCRETE DESIGN 



55 



earth pressure from coming onto the form or concrete before it 
has time to harden properly so as to withstand this pressure. 

Fig. 25, page 53, shows a cross section of a wall requiring 
different character of form work for the two sides. The 
form work on the left side is for a cellar excavated in ground 
which is of such a character that it will stand up with a per- 
fectly vertical face. The form work on the right of the figure 
shows a method for constructing the forms where no spread 
footing is required for the wall. 



Form He/d hy - 




l^^^^/f^T^f^^rr^^^I^^^Wf^y^^ 



Fig. 26. — Wall Form Wired and Bolted Together. (See page 55.) 



Fig. 26, page 55, shows wall form for a building where the 
walls are high and the forms are built in place. 

Wall Above Cellar or Basement. Concrete walls above the 
cellar or basement walls can generally be made about 6 inches 
in thickness and should be reinforced mth \ inch diameter 
bars placed both vertically and horizontally. Place the ver- 
tical bars 18 inches apart and horizontal bars 12 inches apart. 
Place additional bars at corners and diagonally across ' the 
corners of all openings. Fig. 27, page 56, shows a sectional 
wall form as well as a form built in place which can be used 



56 



PLAIN AND REINFORCED CONCRETE 




CONCRETE DESIGN 



57 



for high walls. The sectional wall forms use less lumber and 
hence are more economical. Note that in the form work built 
in place that the lumber is not cut at the corners. 




Fig. 29. — Placing Forms for Concrete Sidewalk. Continuous Mixer in 
Foreground Used for Mixing the Concrete. 




Fig. 30. — Placing the Concrete Base for Sidewalk. 

Small Reinforced Concrete Structures. The plans for re- 
inforced concrete houses, barns or other large structures should 
be given to an engineer or architect experienced in reinforced 
concrete to prepare, for material and labor can both be saved 



58 



PLAIN AND REINFORCED CONCRETE 



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CONCRETE DESIGN 



59 



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60 



PLAIN AND REINFORCED CONCRETE 





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CONCRETE DESIGN 



61 



by so doing besides preventing a calamity in which one or 
more people might be killed. 

The table following gives the dimensions and reinforcement 
of beams and slabs for varying lengths of span, beam spacing 
and floor loading. The rules at the bottom of the table should 
be very carefully studied as they give very important directions 
as to how to bend and place the reinforcing bars. 



MATERIALS FOR BASE OF CELLAR 


FLOORS OR SIDEWALKS 






PER 100 SQUARE FEET OF SURFACE 










£ 
o 

> 






Proportions 








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to 




1:2: 


4 


1 


: 3 : 


6 




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03 

1 

4) 

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Sand 


Gravel 


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B 


Sand 


Gravel 




8 


'BV, 


X^ 
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Kg 

G 
O 


S CO 


One Horse 
Loads 


3'H 


In. 


Cu. 
Yds. 


0>H 


3 


0.93 


6 


3 
4 


0.43 


n 


1 
0.86 


4i 


f 


0.45 


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0.91 


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1.23 


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3.42 

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1.79 


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3.62 



A one horse dump w^agon is assumed to hold 18 cubic feet as a large average 
load. 

Standard vridths of walks, except where special conditions govern, are: 3M, 4, 5 
and 6 feet. 

The concrete slab and the beam below constitutes the floor 
and must therefore be poured at the same time and not inde- 
pendently. On account of the steel in the beams and the nar- 
rowness of the beams only small size broken stones or gravel 
not over one inch in diameter should be used. 

Sidewalks. Concrete sidewalks are composed of two parts, 
namely, the base and the topping. The concrete base can 
generally be made with a 1 : 3 : 6 mix and be from 3 to 5 inches 
thick. The topping or finishing should be about 1 inch thick 
and made of one part Portland Cement to one or two parts of 
clean, coarse sand or crushed stone screenings. The amounts 
of materials for 100 square feet of concrete base and topping 
are given in tables on pages 61 and 62. 



62 



PLAIN AND REINFORCED CONCRETE 



Foundations for Sidewalks. Before laying the concrete 
base prepare a foundation for it of porous material like cinders 
or screened gravel. Make this porous foundation from 2 to 
10 inches thick, depending, of course, upon the kind of soil and 
the climate. No foundation is required where the soil is porous 
and the climate is mild, but no chances should be taken, for 
walks are very frequently ruined by water freezing in the 
foundations and heaving them out of position. 



MATERIALS FOR SURFACE OF CELLAR FLOORS OR SIDEWALKS 
PER 100 SQUARE FEET 







Proportions 


®6D 


o 
> 


1 : 1 


1 : 2 




B 

O 


Sand 


5£ 

P 

B 


Sand 


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r 


3^ 


Inches 


Cu. Yds. 


^ C3 

0>H 


§ 


0.15 


3i 


4 


0.12 


2i 


i 


0.15 


1 


0.23 


5 


^. 


0.18 


3i 


\ 


0.24 


1 


0.31 


6i 


■: ■ 


0.24 


41 




0.32 



For ordinary use concrete sidewalks should be made about 
3 ft., 6 inches wide, and have a joint made every 5 feet across 
the walk. This joint must be through the whole walk, and in 
order to insure a good joint only every other section of walk 
should be laid and after those have hardened then lay the inter- 
mediate ones. Give the top of the walk a slight incline so as 
to drain off the walk easily. Give the walk about a % inch 
slope to every foot width of walk. 

Retaining Walls. The design of the retaining walls shown 
in Fig. 31, page 63, is known as the gravity section, which means 
that the earth pressure is resisted by the weight of the wall. 
The table on page 64 gives the amount of materials per 10 feet 
of length of wall. The amount of material is figured, assum- 
ing that the concrete is made of 1 part Portland Cement to 
2>^ parts of clean, coarse sand, to 5 parts of screened gravel 
or stone. 

The exposed side or face of the retaining wall can be finished 
off in the same manner as described in Chapter VII. The top 



CONCRETE DESIGN 



63 



surface must not be plastered or it will crack and is likely to 
peel off. The surface should be smoothed off with a trowel 
when the concrete is first laid, then as soon as it has begun to 
stiffen scrape off any light-colored scum with a wire brush, wet 
slightly, and trowel it, preferably with a wood float, but using 
no fresh mortar. 



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Fig. 31. — Cross Sections of Retaining Walls. (See page 62.) 

Dams. Dams over 5 feet above the bed of the stream, should 
not be attempted without consulting an engineer and have 
him design it and look after the construction. 

A small concrete dam for an ice pond or a pond for water- 
ing stock may be built across a brook without difficulty. In 
order to take care of the water while constructing the dam, 
dig a temporary trench so as to carry the water around the dam 



64 



PLAIN AND REINFORCED CONCRETE 



while it is being built. If this cannot be done, run the water 
through a wooden trough in the middle of the dam, and after 
the wall, each side of it, is finished, carry the forms across the 
opening, and make these tight enough so that there is no flow 
of water between them; then place the concrete as described 
on page 27. 

Dig a trench across the stream slightly wider than the widths 
of the base of the dam, carrying it down about 18 inches or 
2 feet below the bed of the brook, or if the ground is soft, deep 

MATERIALS FOR RETAINING WALLS 
Proportioxs 1 : 2i : 5 



For retaining walls up to 6 feet, make foundations -2 feet deep; from 6 
to 8 feet, make foundation 3 feet; and 8 to 10 feet, foundation 4 feet under 
groimd. 

Quantities are for each 10 feet of Retaining Wall. 




Sand 



Gravel 






Sq.Ft 



3 rz 









A 


3^2 


11.9 


Go 


lOi 


5.7 


20| 


11.4 


B 


18 


6.7 


38 


; 5i 


3.2 


111 


6.4 


C 


8 


2.96 


17 


i 01 

I ^2 


lA 


5i 


2.8 



The average load for a one horse dump cart is 15 cubic feet (approx.) 

enough to reach good, hard bottom. In case the earth is firm 
enough for a foundation, but is porous either under the dam or 
each side of it, sheet piling consisting of 2-inch tongued-and- 
grooved plank can be pointed and driven with a heavy wooden 
mallet so as to prevent the water flowing under or around the 
dam. Build the forms so as to make the wall of the dimen- 
sions shown in the table. Wet them thoroughly, then mix 
and place the concrete as described in Chapter IV. 

Use 1 part Portland Cement to 2 parts clean, coarse sand 
to 4 parts screened gravel or broken stone. 

Take special care to make the concrete water-tight by using 
a wet mix. Lay the entire dam on one day, if possible, not 
allowing one layer to set before the next one is placed. If it 
is necessary to lay the concrete on two different days, scrape 



CONCRETE DESIGN 



65 



off the top surface of the old concrete in the morning, thoroughly 
soak it with water, and spread on a layer about | inch thick of 
pure cement of the consistency of thick cream, then place the 
fresh concrete before this cement has begun to stiffen. 

Brace the forms on the lower side of the dam well so that the 
forms on the upstream side may be removed in three or four 
days, and the pond allowed to fill. The forms on the down- 
stream face should be left in place well braced for two or three 
weeks. 



MATERIALS FOR SMALL DAMS 



Proportions 1:2:4 



Assumed gravel bottom. 

Quantities are for each 10 feet of dam. 



i| 




Is 
to 




■M 

c 
o 

s 

6 


Sand 


Gravel 


^i 


^ CO 




Eg 

S 


•II 


Ft. 


Ft. 


Ft.-In. 


Cu.Yds. 


Bags 


5^ 


4 


3 


r-r 


1.65 


Hi 


u 


0.8 


3 


1.6 


5 


4 


l'-6" 


2.78 


18i 


2i 


1.4 


5 


2.8 


6 


5 


I'-IO" 


4.18 


29 


3f 


2.1 


n 


4.2 


7 


6 


2'-3'' 


5.84 


39i 


5i 


2.9 


m 


5.8 



The average load for a one horse dump cart is 15 cubic feet (approx.) 

Fence Posts. Concrete fence posts should be made a little 
larger than wood fence posts, and may be either straight for 
the whole length or slightly tapering. For general purposes 
make the post 5 or 6 inches square at the bottom and 4 
or 4 inches square at the top, or for convenience in molding 
they may not be made exactly square, say, 6 inches by 5 
inches at the bottom, and 5 inches by 4 inches at the top. 

As a very slight heaving of a fence post by frost is not ob- 
jectionable it is not necessary to place them in the ground 
more than 2}^ feet, although if for any reason they should be 
absolutely rigid the lower end should go below frost line, which 
in the Northern States is as much as 4 feet down. The length 
of the post is determined by the height which is desired above 
the ground. 



66 



PLAIN AND REINFORCED CONCRETE 



Posts may be built separately, that is, in a separate form 
laid on the ground, but the cheapest way is to build gang forms 
or molds for a number of posts so that several can be molded at 
the same time. The gang molds save considerable lumber, as 
one intermediate board serves as a side to two posts. With 
this method of construction the least amount of ground area 
is also required for molding the posts, and no bracing is neces- 
sary to support the boards for the sides of the posts. Triangular 
1-inch bevel strips should be placed on all edges, which will 
give the posts a neat and pleasing appearance. These bevel 




Fig. 32. — Concrete Gate Posts. 

strips can be obtained readily from a mill, or they may be 
sawed from a 1-inch board by ripping the board lengthwise. 
If desired, the top of the post can be finished with a taper by 
simply insertiiig a triangular block in the top of the mold. 
Never plaster the top of any post. 

Use 1 part Portland Cement to 2 parts clean, coarse sand to 
4 parts broken stone or gravel, for all concrete posts. Make 
concrete posts as follows : Grease or oil the form and fill the 
bottom of the form with concrete to a depth of 1 inch, upon 
which place immediately two pieces of ^-inch round or steel 
rods or No. 6 wire 1 inch in from each side and running the full 
length of the post. Then quickly fill the form to within 1 inch 
of the top with concrete, tamping the wet concrete slightly to 



CONCRETE DESIGN 



67 



drive out any air bubbles. Next place two more rods or wires, 
each 1 inch from each side and fill in the rest of the concrete, 
spading the faces of the posts next to the form boards to leave 
a smooth surface, and lightly trowel the top surface. The 
end boards and the boards between the posts must not be re- 
moved until the concrete is hard and the posts should not be 
handled or moved for at least ten days without danger of 
cracking them. They should be left for three or four weeks 
at least before using and kept damp by sprinkling. The sur- 
faces of the posts do not need to be finished off in any special 
way, for they should be smooth enough without. • 




Fig. 33. — Column Reinforcement, Showing Method of Attaching Hoops. 

For fastening fence wire to the posts, the following method 
is suggested: Take a piece of No. 12 copper wire 12 inches long, 
bend it in two and twist the halves together, leaving the ends 
free for about 2 inches (these should be made beforehand). 
While the concrete is being placed in the forms set two or three 
of those copper wires in the concrete the proper distance for 
stringing wires so that they will be inbedded in the post about 
4 inches and leave the two ends to project from the post about 
2 inches. 

Another very good method is to get a number of ^-inch or 
1-inch round rods or w^ood dowels 6 or 8 inches long and place 
them vertically in the form the proper distance apart for string- 
ing wires. To hold them in place nail a strip of wood across the 
top of the form beside the rod and drive a nail into this strip 



68 



PLAIN AND REINFORCED CONCRETE 



and bend the nail around the rod so as to hold it up against 
the strip. The rods should be well greased and left in the con- 
crete about 1 day, when they can be removed. If they are 
not well greased it will be almost impossible to remove them 
without injuring the concrete. Through the holes the fence 
wire can be strung, or a short piece of wire can be run through 
and the ends twisted around the running fence wire. 

There are several other methods of providing the same means 
of attaching the fence wire to the posts. For instance, insert in 
place of the copper wire described above a galvanized screw 
eye and run the fence wire through it or attach it to the screw 
eye by means of wires. 




Fig. 34. — Beam or Girder Reinforcement, Showing Bent Bars and Stirrups 



7:^::y%'^'':r^-s^mmM 



LIBRARY OF CONGRESS 




